1. Field of the Invention
The present invention relates generally to a binding apparatus for binding loose sheets to form a book-like unit and, more particularly, to a power supply circuit for controlling a binding apparatus.
2. Description of the Related Art
An electrically heated binding apparatus is disclosed in U.S. Pat. No. 4,855,573. The binding apparatus includes a housing with a base connected by a hinge to a cover in which is formed a binding compartment into which a binder cover with sheets to be bound are inserted. The bottom of the binding compartment is provided with electrical contacts which bear against contacts on the binder cover when it is inserted into the binding compartment. The binding cover includes a heat-activated adhesive and an electrical resistance strip to heat the adhesive. An electrical control circuit, which includes timing circuitry for applying binding current for a predetermined length of time, is within the binding apparatus connected to the electrical contacts in the binding compartment. The application of the current is also known as the binding cycle, which is of a set duration.
The electrical resistance strip in the binder covers, or booklets, commonly used, for example, in the known binding apparatus are generally of a low resistance value, for example, of 7 ohms. Since such binder covers are mass produced, a range of resistance values of, for example, 4 to 9 ohms occurs, which although small in an absolute sense is quite large in a relative sense due to the low resistance values, thus resulting in a wide percentage range in the resistances among the individual binder covers. The large percentage change in the resistance causes a corresponding large percentage change in the power drawn by the resistance strip, which in turn results in unpredictable binding. For instance, lower resistance values draw more power so that greater heat is generated by the heating element in the binder cover, which results in discoloration, scorching, and occasionally in burning of the binder cover or vaporization of the adhesive. Higher resistance values for the heating elements draw less power so that, in some instances, the pages are not completely bound in the binder cover.
Another problem which occasionally occurs as a result of, for example, mass produced binder covers is that electrical connections between the contacts and the resistance strip or the continuity of the strip itself may be weak or even lacking altogether so that proper binding of the pages does not occur in the binding cycle. While the known apparatus will detect an open circuit that occurs during the binding cycle, a weak connection is not detected until failure of the connection at some point during the binding cycle. This can result in partially bound books, which can be worse than no binding at all.